Image suppression circuit



ct. 26, 1937. R. B. ALBRIGHT 2,097,359

IMAGE SUPPRESS lON CIRCUIT Filed April 27, 1936 T UTOMFlT/C VOLUME CONT/EOL 0@ Q/lD B/HS SUPPLY Patented oa. as, 1937 1, Y C 2,097,359

Aram OFFICE EMAGE SUPPRESSION CIRCUIT Robert 1B. Albright, Philadelphia, Pa., assigner to Philco Radio n Television Corporation, Philadelphia, Pa., acorporation of Delaware Application April 27, 1936, Serial No. 76,673

6 Ciaims. (Cl.'250-20) This invention relates to novel signal transfer quency is 450 k. c., the image signal frequency means for radio receivers and like systems whereband will extend from 750 to 1200 k. c. in it is desired to transfer signals of certain fre- The present invention, when applied to a superquency and to reject signals of certain other freheterodyne radio receiver, is adapted to suppress quencies. More particularly, the invention reor reject the undesired image signals, such as 5 lates to a novel circuit for a superheterodyne those above mentioned. For the purpose of the radio receiver by means of which undesired sigpresent specification and appended' claims, an nals, commonly known as image signals, may image or undesired signal may be defined as a be eifectively suppressed white desired signals signal other than the one it is desired to receive 10 may be transmitted substantially unaffected. which will heterodyne with the local oscillator 10 One object of the invention, therefore, is to signal or a harmonic thereof to produce a signal, provide novel signal transfer means adapted to at least one component of which is of the intertransfer desired signals and to reject undesired mediate frequency, or thereabouts. signals. The circuit shown in the drawing is particu- Another object of the invention is to provide a larly adapted to select signals in the long wave 15 novel circuit which is adapted to receive signal band, that is signals having a frequency in the energy either from a high impedance source, such general region 150 to 300 kilocycles, and to attenas a conventional antenna, or from a low impeduate undesired signal-s in the frequency region ance source, such as a transmission line as-soci- 750 to 1200 kilocycles. It will be Vnoted that such Y ated with certain well-known antennae. suppression must be highly effective because the 20 A further object of the invention is to provide undesired signal region includes the more powera novel capacitive Coupling between an input cirful commercial broadcast stations, while the cuit and a tuned circuit, by means of which image range of desired signals include stations which, signals may be effectively attenuated. in general, transmit signals with less power than The invention is particularly adapted for use those in the frequency range of the undesired in a superheterodyne radio receiver to suppress or signals. The circuit of the present invention is reject the image signals commonly encountered adapted to suppress effectively undesired signals in such a system, as above mentioned. For the emanating from powerful transmitting stations purpose of disclosure, therefore, the invention will without aiecting desired signals from less power- SO be described with particular reference to this ful stations. i i adaptation, out it willpbe understood that the Referring now to the drawing, signals generinvention is not thus limited in scope. ated by the local oscillator, indicated by refer- In the accompanying drawing, the single figure ence character O, may be applied to a control is a diagrammatic iliustration of a preferred grid g of vacuum tube V, whose output circuit 5 form of the invention as applied to a superhetmay be connected to utilization means including y35 erodyne radio receiver. a conventional intermediate frequency amplier, Asis well known, in a superheterodyne receiver, detector, audio frequency amplifier, and V10nd the desired signal is combined with a signal from speaker or equivalent device. The signal which a local oscillator producing signals having freit is desired to receive may be selected by the quencies equal to the sum and dierence of the means described hereinafter and supplied to a 40 desired signal frequency and the oscillator frecontrol grid g1 of the tube V. The mixing or quency. The difference frequency signal, having heterodyne action may take place within the tube the intermediate frequency, is selected and am- V. The means for selecting,v the desired signal r pliiied. The oscillator frequency is thus the deand for attenuating the undesired signal comsired signal frequency plus the intermediate freprises a tuned circuit L, C and C1 whichcircuit quency. Likewise, undesired image signals, may be tuned by means of the variable condenser whose frequency is equal to the oscillator fre- C to the frequency of the signal which it is desired queney plus the intermediate frequency, or twice to receive. The variable condenser C may be the oscillator frequency minus the intermediate controllable jointly with the variable condenser of 50 frequency, will, when combined with the'oscilthe oscillator, as well understood. Preferably, lator signal, produce undesired signals of interthe variable condenser C is rconnected in shunt mediate frequency. For example, if the desired with the control grid g1 and to ground, as illus-Y signal frequency band is from 150 to 300 k, c., i. e., trated. The inductance L may be connected to the long wave band, and the intermediate frethe control grid g1 and also to the condenser C1, 55

which condenser may also be connected to ground.

Signals built up across the condenser C1 will be applied through the inductance L to the control grid g1. Signals may be supplied to the condenser C1 by means of the secondary winding S of the input transformer, the other end of the secondary winding being connected to ground through a blocking condenser C2, which condenser may have a very low impedance to signals of radio frequency. A biasing potential, for example, either a iXed bias or an automatic volume control bias, may be applied to the control grid tf1 through the isolating resistor R. The primary winding of the input transformer may be divided into sections P and P1. Winding P1 may be center tapped, as illustrated. Designating the terminals of the primary winding as a, b, c, and d, the impedance looking into P1 between terminals b and d may be low, whereas looking into the transformer from terminals a and d, the impedance may be very high. A switch SW may be adapted to connect either of the terminals c and d to ground, depending upon whether the transformer is connected to a high impedance antenna A or to a low impedance transmission line TL.

When it is desired to supply signals to the receiver by means of the low impedance line, the line may be connected across terminals h and d, and the terminal c may be grounded by means of the switch SW. The input circuit is then balanced to ground, as will be obvious, and noisesignal pick-up of the transmission line will be minimized. When it is desired to supply signals to the receiver from the high impedance antenna A, the antenna may be connected to terminal a, and the terminal d may be connected to ground by means of the switch SW. Preferably the coupling between P and S and between P1 and S should be as high as possible. Signals supplied to the receiver either from the antenna or from the transmission line will appear across the secondary S. In addition, the transformer will also introduce an effective inductive reactance in series with the signal source, due iirst to the effective impedance of the signal collector, which may be either inductive or capacitive, and second to the leakage reactance of the transformer itself. Thus, the transformer serves first to form an effective signal source properly matched to either a high impedance collector or a low impedance signal source which may in addition be balanced to ground, and second the transformer serves to introduce an effective inductive reactance in series with the signal source. The circuit S, C1 may be tuned to be resonant either above or preferably below the frequency range of signals which it is desired to receive, but in any case this circuit should be resonant well below the frequency range of undesired signals. It is apparent that the impedance of an inductive reactance will increase with frequency while that of a capacitive reactance will decrease. Thus, the equivalent impedance of the secondary or equivalent signal source will increase with frequency ever, undesired signals, which may be present at the secondary S, will not build up to an appreciable extent in the second tuned circuit L, C and C1 due to the fact that the impedance of the condenser C1 will be small for signals of this frequency, whereas the equivalent series impedance of the input transformer will be high for signals in this frequency region. Thus, the amplitude of undesired signals transferred to the condenser C1 will bel substantially attenuated by this coupling condenser. The undesired signals will be further attenuated by means of the inductance L whose impedance to signals of this frequency will be very high as compared with either the impedance of condenser C or that of C1. Hence, the selecting circuit serves to attenuate undesired signals, first, by short-circuiting them through the condenser C1, and, second, by the operation of the inductance L to prevent the development of undesired signal voltage across the tuning condenser C.

In one practical embodiment of the invention a circuit having the following constants was found to be highly satisfactory. The inductance L comprised 400 turns of litzendraht wire (3 strands No. 40 B. & S.) wound on a one-half inch form and had an inductance of about 2500 microhenries. The condenser C1 had a value of 0.02 mf. and the condenser C was a variable condenser having a maximum capacity of 385 mmf. The inductance S had a value of about 150 microhenries and comprised turns of No. 38 B. & S. wire on a one-half inch form. The inductance P1 was similar to S and was center tapped at the 50th turn. Inductance P comprised 600 turns of No. 38 wire. All three of the inductances P, P1 and S were closely Wound together using a winding of the universal type on a one-half inch diameter form. The coupling between S, P and P1 was made as high as possible. This particular transformer was suited to match a 100 ohm transmission line connected between terminals b and d, whereas the inductance between terminals a and d was about 9 millihenries and thus was of suiiiciently high impedance to work satisfactorily out of a conventional antenna. The resonant frequency of the circuit S-C1 was about 100 k. c.

It will be seen from the illustration and above description that the novel circuit provided by the invention is adapted to effectively suppress undesired signals in a frequency range including powerful transmitting stations, while permitting the transfer of signals in another frequency range including less powerful transmitting stations. Moreover, the invention makes possible the use of either a high impedance signal source or a low impedance signal source in cooperative association with the novel circuit for suppressing undesired signals. Although the invention has been illustrated and described with reference to a particular preferred embodiment, it will be understood that various changes and modifications may be made without departing from the spirit and scope of the invention.

I claim:

1. In a radio receiving system, a source of desired wave signals and undesired wave signals, said undesired signals having a higher frequency than said desired signals, a space discharge device having an input circuit, means for transferring said desired signals from said source to said input circuit and for minimizing the transfer. of undesired signals comprising means providing an effective inductive reactance in series with said source, purely capacitively reactive inductively reactive means for forming a loopV circuit resonant to a frequency below said undesired wave signal frequency, a resonant circuit including said purely capacitive means and energized only by the signal across said capacitive means, said last-named circuit being tunable to the frequency of said desired wave signal, and a connection between said input circuit and said tuned circuit.

2. In a radio receiving system, a source of desired wave signals and undesired wave signals, said undesired signals having a higher frequency than said desired signals, a space discharge device having an input circuit, means for transferring said desired signals from said source to said input circuit and for minimizing the transfer of undesired signals comprising means providing an effective inductive reactance in series with said source, purely capacitively reactive means serially connected to said source and said inductively reactive means for forming a loop circuit resonant to a frequency below said desired wave signal frequency, a resonant circuit including said purely capacitive means and energized only by the signal across said capacitive means, said last-named circuit being tunable to the frequency of said desired wave signal, and a connection between said input circuit and said tuned circuit. Y

3. In a radio receiving system, a source of desired wave signals and undesired wave signals, said undesired signals having a higher frequency than said desired signals, a space discharge device having an input circuit, means for transferring said desired signals from said source to said input circuit and for minimizing the transfer of undesired signals comprising means providing an effective inductive reactance in series with said source, purely capacitively reactive means serially connected to said source and said inductively reactive means for forming a loop circuit resonant to a frequency below said undesired wave signal frequency, a resonant circuit including an inductively reactive element and said purely capacitive means, said lastnamed circuit being energized only by the signal across said capacitive means and being tunable to the frequency of said desired wave signal, and connections for supplying a signal from said capacitively reactive element to said input circuit.

4. In a radio receiving system, a source of desired wave signals and undesired wave signals, said undesired signals having a higher frequency than said desired signals, a space discharge device having an input circuit, means for transferring said desired signals from said source to said input circuit and for minimizing the transfer of undesired signals comprising a transformer having a primary winding connected to said source and a secondary winding for forming an effective signal source having an effective inductive reactance in series therewith, purely capacitively reactive means serially connected to said eifective source and said inductively reactive means for forming a loop circuit resonant to a frequency below said undesired wave signal frequency, a resonant circuit including said purely capacitive means and energized only by the signal across said capacitive means, said last-named circuit being tunable to the frequency of said desired wave signal, and a connection between said input circuit and said tuned circuit.

5. Ina radio receiving system, a source of desired wave signals and undesired wave signals, said undesired signals having a higher frequency than said desired signals, a space discharge device having an input circuit, means for transferring said desired signals from said source to said input circuit and for minimizing the transfer of undesired signals comprising a transformer having a tapped primary winding, connections for alternatively connecting said source to different portions o-f said primary winding, and a secondary winding for forming an eiiective signal source having an effective inductive reactance in series therewith, purely capacitively reactive means serially connected to said effective source and said inductively reactive means for forming a loop circuit resonant to a frequency below said undesired wave signal frequency, a resonant circuit including said purely capacitive means and energized only by the signal across said capacitive means, said last-named circuit being tunable to the frequency of said desired wave signal, and a connection between said input circuit and said tuned circuit.

6. In a radio receiving system, a source of desired Wave signals and undesired wave signals, said undesired signals having a higher frequency than said desired signals, a space discharge device having an input circuit, means for transferring said desired signals from said source to said' input circuit and for minimizing the transfer of undesired signals comprising a transformer having -a primary winding connectedrto said source and a secondary winding for forming an effective signal source having an eifective inductive reactance in series therewith, purely capacitively reactive means serially connected to said effective source and said inductively reactive means for forming a loop circuit resonant to a frequency below said desired wave signal frequency, a resonant circuit including an inductively reactive element and Vsaid purely capacitive means, said last-named circuit being energized only by the signal across said capacitive means and being tunable to the frequency of said desired wave signal, and connections for supplying a signal from said capacitively reactive element to said input circuit.

ROBERT B. ALBRIGHT. 

